Method for maintaining an aqueous solution of sodium borate in a liquid form at room temperature

ABSTRACT

A method of keeping an aqueous solution of sodium borate liquid at a storage temperature, in which method, in order to pass said solution from an initial temperature to the storage temperature, said aqueous solution of sodium borate is subjected to heat treatment comprising at least one cooling or heating operation at a speed lying in the range 1° C./min to 100° C./min, to reach a holding temperature lying in the range −50° C. to +200° C., followed by holding the holding temperature for a time lying in the range 1 s to 100 h, followed by cooling or heating at a speed lying in the range 1° C./min to 100° C./min. Use of the method in a method of supplying hydrogen for a fuel cell.

The present invention relates to the treatment of an aqueous solution ofsodium borate that results from producing hydrogen by decomposing sodiumborohydride, where the hydrogen is intended for a variety ofapplications, and in particular for feeding a fuel cell in a motorvehicle.

For a variety of reasons, such as seeking energy independence, reducingpollution, reducing the emission of greenhouse gases, or in order toeconomize hydrocarbon resources, attempts are being made to developmethods of producing energy from hydrogen. Such methods rely inparticular on using fuel cells in which hydrogen reacts with anoxygen-containing gas, thereby producing electricity.

Any development of such techniques relies on effective and safetechniques being developed for storing hydrogen. For example it is knownto store hydrogen in the form of a compressed gas, and it is also knownto store hydrogen in the form of a liquefied gas. But neither of thosetechniques completely satisfies the constraints of the automobileindustry, in particular, even though it is seeking to be able to fitvehicles with fuel cells that are powered with hydrogen. Storing gas inthe form of a compressed gas is very bulky and can lead to problems ofsafety because of the high pressures used. Liquid storage also leads todrawbacks, firstly because liquefying gas requires a large amount ofenergy to be used, and secondly because handling liquid hydrogen atextremely low temperatures involves risks that make it difficult toapply liquid hydrogen to the automobile industry.

The problem of producing and storing hydrogen under conditions ofsatisfactory safety also arises in other fields of application forhydrogen, for example in the medical field, in the agrifood business, orin various heat treatments.

In order to remedy those drawbacks, proposals have been made to storehydrogen in motor vehicles in the form of sodium borohydride, and toproduce hydrogen on demand by decomposing the sodium borohydride byreaction with water to produce firstly hydrogen and secondly a residueconstituted by an aqueous solution of sodium borate. That technique hasthe advantage of enabling hydrogen to be stored in a manner that is safeand of producing hydrogen in a manner that is convenient for feeding toa motor vehicle fuel cell, however it suffers from a drawback. Theaqueous solution of sodium borate is recovered and stored in a tankwhich needs to be emptied regularly. Unfortunately, the sodium boratesolution which is liquid at the temperature at which water reacts withsodium borohydride (in the range 100° C. to 180° C.), tends tocrystallize on cooling down to ambient temperature, thus making itdifficult to empty the sodium borate tank.

The object of the present invention is to remedy that drawback byproposing means for conserving sodium borate solution derived fromhydrogen production by decomposing sodium borohydride liquid at ambienttemperature.

To this end, the invention provides a method of keeping an aqueoussolution of sodium borate liquid at a storage temperature, in whichmethod, in order to pass said solution from an initial temperature tothe storage temperature, said aqueous solution of sodium borate issubjected to heat treatment comprising at least one cooling or heatingoperation at a speed lying in the range 1° C. per minute (° C./min) to100° C./min, to reach a holding temperature lying in the range −50° C.to +200° C., followed by holding the holding temperature for a timelying in the range 1 second (s) to 100 hours (h), followed by cooling orheating at a speed lying in the range 1° C./min to 100° C./min.

Preferably, the heat treatment includes at least two holding operationsat different holding temperatures.

Prior to performing the heat treatment, the aqueous solution of sodiumborate is at an initial temperature lying in the range 100° C. to 180°C., and after the heat treatment has been performed, the aqueoussolution of sodium borate is at a storage temperature lying in the range−50° C. to +300° C., and preferably in the range −20° C. to +50° C.

Preferably, the aqueous solution of sodium borate contains 5% to 65% byweight of sodium borate and may also contain 0% to 10% by weight ofsoda.

The invention also relates to a method of generating hydrogen in whichsodium borohydride is caused to react with water, and there areseparated out therefrom firstly a mixture constituted mainly byhydrogen, and secondly an aqueous solution of sodium borate, in whichthe aqueous solution of sodium borate is subjected to the heat treatmentof the invention.

The method can be used for feeding a fuel cell with hydrogen.

The fuel cell is preferably the fuel cell of a motor vehicle.

The method can also be used for generating hydrogen used in particularin medicine, in the agrifood business, in electronic componentfabrication, or in performing heat treatments on metal products.

The invention is described below in greater detail in non-limitingmanner and as illustrated by an example.

In novel and unexpected manner, the inventors have found that an aqueoussolution of sodium borate derived from the method of producing hydrogenby catalytic decomposition of an aqueous solution of sodium borohydrideand containing a small fraction of soda retains its fluidity even whensubjected to heat treatment consisting in a string of sequences ofcooling and/or heating separated by holding at temperature levels. Thecooling or heating operations need to be performed at heating or coolingspeeds lying in the range 1° C./min to 100° C./min, and preferably lessthan 50° C./min, and better still less than 20° C./min. The temperaturesof the levels that are held must lie in the range −50° C. to +200° C.,and the length of time for which these levels are held must lie in therange 1 s to 100 h, preferably in the range 10 s to 100 h, morepreferably in the range 10 s to 50 h, and better still in the range 30 sto 2 h. The cooling or heating speeds, the temperature levels, thedurations of the levels, and the order in which sequences are performedall constitute parameters that enable the method to be controlled. Themethod is used for taking an aqueous solution of sodium borate producedat an initial temperature and bringing it to a storage temperature. Thestorage temperature lies in the range −50° C. to +300° C., andpreferably in the range −20° C. to +50° C. These preferred temperaturescorrespond to the temperatures that can occur in a tank of a motorvehicle that remains outside, depending on the season and the location.

By way of example, the following treatment has been performed on anaqueous solution of sodium borate that was obtained by producinghydrogen by decomposing sodium borohydride by reaction with water so asto feed a fuel cell with hydrogen:

the aqueous solution of sodium borate was at a temperature of 135° C. atthe outlet from the reactor for decomposing sodium borohydride;

the solution was initially cooled down to a temperature of 80° C. at aspeed of 5° C./min;

the aqueous solution of sodium borate was held at a temperature of 80°C. for 12 h;

then the aqueous solution of sodium borate was cooled down to atemperature of 60° C. at a speed of 5° C./min;

the aqueous solution of sodium borate was then held at the temperatureof 60° C. for 8 h;

then the aqueous solution of sodium borate was cooled to a temperatureof 40° C. at a speed of 5° C./min;

the solution of sodium borate was then held at the temperature of 40° C.for 15 h; and finally

the aqueous solution of sodium borate was brought to ambienttemperature, i.e. about 20° C., at a speed of about 5° C./min.

Following that heat treatment, the aqueous solution of sodium borate didnot present any crystallization and remained in the form of a viscousliquid. The solution obtained in that way was easy to handle and couldbe extracted from the tank for storing the aqueous solution of sodiumborate without any difficulty.

The method is particularly adapted to treating the aqueous solution ofsodium borate that results from the method of producing hydrogen for avariety of uses and more particularly for feeding on demand the fuelcell of a motor vehicle.

In a motor vehicle including a fuel cell fed with hydrogen bydecomposing sodium borohydride, the sodium borohydride is stored in theform of a liquid solution in a tank. That solution has a concentrationby weight of sodium borohydride lying in the range 5% to 35%, andpreferably in the range 15% to 25%. The solution may further include acontent lying in the range 0% to 10% by weight of soda and preferably0.5% to 4% that is added to stabilize the aqueous solution of sodiumborohydride. Although such an addition is the usual practice, it is notessential. When the vehicle requires electricity to be produced, thesodium borohydride in aqueous solution is taken from the fuel tank andsent to a catalytic reactor where it is decomposed by being reacted withwater, thereby producing both hydrogen and sodium borate. The reactiontakes place at a temperature lying in the range 100° C. to 180° C. andpreferably above 110° C. and better above 130° C., but below 150° C. andbetter below 140° C. The reaction product is then sent to a gas/liquidseparator which separates the gaseous hydrogen, possibly mixed withwater vapor, from an aqueous solution that contains mainly sodium borateand that is at a temperature likewise in the range 100° C. to 180° C.,preferably in the range 110° C. to 150° C., and better in the range 130°C. to 140° C.

In accordance with the method of the present invention, the aqueoussolution of sodium borate also contains a little soda, and it is broughtto the storage temperature by heat treatment consisting in a successiveof heating or cooling operations separated by being held at leveltemperatures, as described. At the end of the heat treatment, theaqueous solution of sodium borate is sent to a storage tank in which itremains liquid until it has been emptied out.

As mentioned above, the method can be used in any installation forproducing hydrogen by decomposing sodium borohydride, regardless of theuse intended for the hydrogen that is produced in this way.

1. A method of keeping an aqueous solution of sodium borate liquid at astorage temperature, in which method, in order to pass said solutionfrom an initial temperature to the storage temperature, said aqueoussolution of sodium borate is subjected to heat treatment comprising atleast one cooling or heating operation at a speed lying in the range 1°C./min to 100° C./min, to reach a holding temperature lying in the range−50° C. to +200° C., followed by holding the holding temperature for atime lying in the range 1 s to 100 h, followed by cooling or heating ata speed lying in the range 1° C./min to 100° C./min.
 2. A methodaccording to claim 1, wherein the heat treatment includes at least twoholding operations at different holding temperatures.
 3. A methodaccording to claim 1 or claim 2, wherein, prior to performing the heattreatment, the aqueous solution of sodium borate is at an initialtemperature lying in the range 100° C. to 180° C., and after performingthe heat treatment, the aqueous solution of sodium borate is at astorage temperature lying in the range −50° C. to +300° C.
 4. A methodaccording to claim 3, wherein the storage treatment lies in the range−20° C. to +50° C.
 5. A method according to claim 1, wherein the aqueoussolution of sodium borate contains 5% to 65% by weight of sodium borate.6. A method according to claim 5, wherein the aqueous solution of sodiumborate further contains 0% to 10% by weight of soda.
 7. A method ofgenerating hydrogen in which sodium borohydride is caused to react withwater and both a gaseous mixture constituted mainly of hydrogen and anaqueous solution of sodium borate are extracted therefrom, wherein theaqueous solution of sodium borate is subjected to the method accordingto claim
 1. 8. The use of the method of claim 7, comprising feedinghydrogen to a fuel cell.
 9. The method of claim 8, wherein the fuel cellis the fuel cell of a motor vehicle.
 10. The method according to claim 7wherein the hydrogen is used in medicine, in the agrifood industry, inthe fabrication of electronic components, and/or in the implementationof heat treatments on metal products.